1. Field of the Invention
The present invention relates to a charging apparatus that is provided in electronic devices such as mobile devices and that charges a battery by receiving power from a DC power supply such as an AC adapter.
2. Description of Related Art
In recent years, electronic devices such as mobile devices having batteries have charging apparatuses which use constant current charging control and constant voltage charging control as a method of controlling battery charging. Particularly, lithium-ion batteries have high energy density per unit volume and per unit mass, and enable reduction in size and weight of devices having the lithium-ion batteries. The schemes used for charging a lithium-ion battery include a constant voltage charging scheme that supplies charging current while maintaining a constant voltage of the battery, and a constant current/constant voltage charging scheme that charges the battery with a constant voltage after charging the battery with constant current. Both charging apparatuses adopting the above-described schemes detect that charging current becomes equal to or less than predetermined full charging current upon constant voltage charging and finish charging.
FIG. 1 is a circuit diagram showing a configuration of a conventional charging apparatus.
In FIG. 1, charging apparatus 10 adopts a configuration including: AC adapter input terminal 11 that receives the output DC voltage of an AC adapter; battery 12 such as a lithium battery; load circuit 13; charging section 14 configured with bipolar transistor Q1 and current detecting resistance Rs; current difference amplifier 15 that amplifies the voltage detected by current detecting resistance Rs; voltage difference amplifier 17 that amplifies the voltage difference between the battery voltage and the reference voltage generated by reference voltage source 16; voltage difference amplifier 19 that amplifies the voltage difference between the output voltage of current difference amplifier 15 and the reference voltage generated by reference voltage source 16; comparator 21 that compares the battery voltage with the reference voltage generated by reference voltage source 20; and switch 22 that selects one of the output (e) of voltage difference amplifier 19 and the output (d) of voltage difference amplifier 17 and applies the result to the base of bipolar transistor Q1.
Various types of electronic circuits in the electronic device provided with a charging apparatus are collectively referred to as load circuit 13, and load circuit 13 is connected so that power is supplied from battery 12. Further, the AC adapter shows constant current drooping characteristics.
Current detecting resistance Rs of charging section 14 detects current flowing into battery 12. The detected voltage is amplified by current difference amplifier 15. Bipolar transistor Q1 controls charging current flowing into battery 12 at constant current when the output of voltage difference amplifier 19 is selected for the base, and controls the battery voltage at a constant voltage when the output of voltage difference amplifier 17 is selected for the base. FIG. 2 is a timing chart that illustrates the constant current charging control and constant voltage charging control operations of the charging apparatus in FIG. 1, and shows the AC adapter voltage when the AC adapter is connected to AC adapter input terminal 11, the battery voltage, the charging current, the output voltage (d) of voltage difference amplifier 17, the output voltage (e) of voltage difference amplifier 19, the output voltage (f) of comparator 21 and the amount of heat (g) produced in bipolar transistor Q1.
Io is current flowing into load circuit 13, and Ibat is current flowing into battery 12. The solid lines in FIG. 2 show charging characteristic 1 (Io=Ibat) when no or little current is supplied to load circuit 13, and the dotted lines in FIG. 2 show charging characteristic 2 (Io>>Ibat) when large current is supplied to load circuit 13. Further, periods t1 and t2 in FIG. 2 are divided into t1-i and t2-i, and t1-ii and t2-ii according to the weight of the load on load circuit 13. Periods t1 and t2 are divided into t1-i and t2-i in the case of charging characteristic 1, and divided into t1-ii and t2-ii in the case of charging characteristic 2.
The operation of above-described charging apparatus 10 in the case of charging characteristic 1 (Io=Ibat) where little current is supplied to load circuit 13, will be described in detail using the timing chart of FIG. 2.
[Period t1-i]
The AC adapter voltage is inputted to AC adapter input terminal 11, charging current increases, the AC adapter voltage decreases according to the constant current drooping characteristics of the AC adapter, and the battery voltage increases. In this period t1-i, the battery voltage still remains low, and so the output voltage (d) of voltage difference amplifier 17 becomes low.
When above (d) still remains low and (a) and (b) of switch 22 are connected, the output voltage (f) of comparator 21 becomes low by the connection of (a) and (b) of switch 22. By this means, charging apparatus 10 operates by constant current charging control by the AC adapter.
Fixed charging current flows, and so the output voltage (e) of voltage difference amplifier 19 becomes low.
In this period, by a small voltage difference between the AC adapter voltage and the battery voltage and the fixed level charging current based on constant current dropping characteristics of the AC adapter, low heat as shown in FIG. 2(g) is produced due to the power loss of bipolar transistor Q1 until the battery voltage reaches a predetermined voltage.
Although a current value based on the constant current drooping characteristics of the AC adapter is used as charging current, when the AC adapter has high current supply capacity and the charging current based on the constant current drooping characteristics is too large, charging apparatus 10 is charged with constant current following the constant current control circuit in the charging apparatus.
[Period t2-i]
The battery voltage reaches the desired voltage, the AC adapter voltage returns to a predetermined voltage that does not cause droop, the charging current decreases, and the charging of the battery is completed. In this period t2-i, the battery voltage is almost fully charged, and so the output voltage (d) of voltage difference amplifier 17 becomes high.
When above (d) is high and (a) and (b) of switch 12 are connected, the output voltage (f) of comparator 21 becomes high. In FIG. 2, charging apparatus 10 operates by constant voltage charging control.
The charging current decreases, and so the output voltage (e) of voltage difference amplifier 19 increases.
Decreasing charging current in a state where a voltage difference between the AC adapter voltage and the battery voltage is high, results in a transition from a high heat produced state as shown in FIG. 2(g) to a low heat produced state due to the power loss of bipolar transistor Q1.
Apparatuses that perform the above-described charging control include the charging control circuit disclosed in Patent Document 1 (Japanese Patent Application Laid-Open No. 2003-348766), for example. The charging control circuit disclosed in Patent Document 1 switches the desired voltage for battery voltage by an external switch when the battery is charged using constant current charging control and constant voltage charging control.
However, such a conventional charging apparatus has the following problems.
If an electronic device is operated while the battery is being charged, heavy load is imposed on the battery. With charging apparatus 10 shown in FIG. 1, in constant voltage charging control, heat produced due to the power loss of bipolar transistor Q1 which is the charging transistor, poses a serious problem. Although current detecting resistance Rs is used to detect the amount of current flowing into the battery, if load is heavy while the battery is being charged, with the conventional configuration, it is not possible to distinguish between the current flowing into the battery and the load current. Therefore, although the charging of the battery is completed, the power supply path from the AC adapter to the battery is not switched, and consequently power is continuously supplied from the AC adapter, and the power loss by the charging transistor occurs continuously.
The case of charging characteristic 2 (Io>>Ibat) where large current is supplied to load circuit 13 in the constant voltage charging operation of charging apparatus 10 shown in FIG. 1 will be described using the timing chart of FIG. 2. In addition, the period for charging characteristic 2 is shown as period t1-ii and t2-ii. 
[Period ti-ii]
The AC adapter voltage is inputted to AC adapter input terminal 11, the charging current increases, the AC adapter voltage decreases, and the battery voltage increases. As a result of Io>>Ibat, it takes the battery voltage a longer time to increase than in the case of charging characteristic 1.
[Period t2-ii]
Although the charging current decreases, the AC adapter voltage returns to a predetermined voltage, the battery voltage reaches the desired voltage, and the current flowing into battery 12 Ibat decreases, the current supplied to load circuit 13 continues flowing. In this period, by a large voltage difference between the AC adapter voltage and the battery voltage and the current supplied to load circuit 13, a high heat produced state as shown in FIG. 2(g) continues due to the power loss of bipolar transistor Q1.
In order to endure this heat production, bipolar transistor Q1 needs to be configured with components having high heat resistance and be implemented on a board that is designed to release heat. This causes an increase in cost. Further, in limited space of mobile devices, or the like, this causes a substantial increase in area for implementing components.